The IEEE802.16e system is part of a global standardization for broadband wireless access systems. IEEE802.16e does not have a hierarchical structure of HLR, VLR, MSC, BSC, RNC, etc. unlike second and third generation mobile communications systems, but does include a mobile subscriber station (MSS), a base station (BS), and an authentication service authorization server (ASA server). A physical (PHY) layer and a medium access control (MAC) layer are defined between the BS and the MSS.
FIG. 1 is a block diagram illustrating a related art broadband wireless access system in which a handover operation is performed on a data plane. FIG. 2 is a block diagram illustrating a related art broadband wireless access system in which a handover operation is performed on a control plane.
Referring to FIG. 1 and FIG. 2, a handover in a broadband wireless access system is performed according to IEEE802.16e, as explained below.
1. Pre-Processing for Handover
Pre-processing for handover includes broadcasting associated information from a base station to a neighboring base station to inform all mobile subscriber stations within a respective cell of the associated information (Network Topology Advertisement). The pre-processing also includes measuring a channel quality of the neighboring base station based on the associated information (MSS Scanning of Neighboring BS). The pre-processing also includes matching parameters between the base station and the neighboring base station, such as for example, an initial mobile subscriber station power, a time difference for synchronization, and other parameters (Association Procedures).
a. Network Topology Advertisement
In network topology advertisement, the base station broadcasts information associated with network configuration to all mobile subscriber stations within a cell via a MOB_NBR-ADV MAC message, thereby providing all mobile subscriber stations within the cell with the information pertaining to neighboring base stations.
b. MSS Scanning of Neighboring BS
In MSS scanning of neighboring BS, a mobile subscriber station (MSS) scans a neighboring base station (BS) and requests a scanning interval for scanning the neighboring BS from the BS via a MOB_SCN-REQ MAC message. The BS then transmits a MOB_SCN-RSP MAC message in response to the request to assign an interval for scanning (scanning interval) the neighboring BS. Alternatively, the BS may directly transmit the MOB_SCN_RSP MAC message without a request from the MSS (unsolicited response). In the case of an unsolicited response, the scanning interval and an offset unit for scanning start are assigned on a frame unit basis.
c. Association Procedures
During association procedures, the MSS performs a ranging process with a BS to normally join a cell. In the ranging process, the MSS acquires basic information for communications from the BS. In other words, the MSS performs BS scanning to select a new BS and then performs the association procedures. The MSS and the BS transmit a RNG_REQ MAC message and a RNG_RSP MAC message, respectively, to set values such as for example, a power offset value, a timing offset value, and other values of the MSS to suitable values. Transmission of the RNG_REQ MAC message is called an initial ranging operation and is a basic operation that the MSS may perform in a network entry procedure. When a handover is performed, a target BS accepts a new MSS and transmits items associated with the respective cell of the new MSS to the former serving BS and stores information pertaining to the MSS.
2. Handover Process
In a handover (HO) process, the MSS initiates handover with the neighboring BS based on the channel quality information acquired in the handover preprocessing.
a. Cell Selection
FIG. 3 is a diagram illustrating a related art cell selection procedure in a related art broadband wireless access system. In a cell selection procedure, a cell is changed to enable the MSS to make a new registration to a BS, allowing for reception of a signal having an SIN (signal to interference noise ratio) superior to that of a signal transmitted from the BS of the current cell, prior to a normal registration of the MSS to a cell. In such case, since the MSS does not perform the registration procedure, the BS is unable to determine a moving situation of the MSS.
b. HO (handover) Initiation
In HO initiation, a handover may be initiated by the MSS or the BS. When the BS requests (initiates) a handover, the BS transmits a MOB_BSHO-REQ MAC message to the MSS. On the contrary, when the MSS requests a handover, the MSS transmits a MOB_MSSHO-REQ MAC message to the BS.
If the MSS transmits the MOB_MSSHO-REQ MAC message, the MSS preferentially transmits the SINR of a signal received from a neighboring BS to the BS to enable the MOB_MSSHO-REQ MAC message to be transmitted to a current serving BS, The current serving BS then selects a candidate BS as a target BS for performing the handover.
In a case where the BS receives the MOB_MSSHO-REQ MAC message from the MSS or in a case when the BS has not yet transmitted the MOB_BSHO-REQ MAC message to handover the MSS, the BS may allow the handover of the MSS after confirmation of an acknowledgement (ACK) for handing over a specific MSS from the neighboring BS. After receiving the MOB_BSHO/MSSHO-REQ MAC message, the MSS/BS transmits a MOB_MSSHO/BSHO-RSP MAC message to inform the BS/MSS of a target BS for the handover.
c. HO Cancellation
In HO cancellation, after the MSS/BS transmits the MOB_MSSHO/BSHO-REQ MAC message, the MSS may cancel the handover. In canceling the HO, the MSS sets a specific field (HO_Type=01) of a MOB_HO-IND MAC message and transmits the set field to the BS. The BS receives the message and cancels the proceeding handover.
d. Termination with the Serving BS
In termination with the serving BS, the MSS transmits the MOB_HO-IND MAC message to inform the serving BS that the handover is normally completed and then completes the handover operation. In completing the handover operation, the MSS sets a specific field (HO_Type=00) of the MOB_HO-IND MAC message to transmit the information pertaining to the normal handover termination to the serving BS. The serving BS receives the MOB_HO-IND MAC message from the MSS and terminates a MAC state machine allocated to the MSS. The serving BS also terminates an ARQ (automatic repeat request) connection and all connections associated with data transmission.
e. HO Rejection
In HO rejection, the MSS is capable of rejecting a handover recommended by the BS. In rejecting a handover, the MSS sets a specific field (HO_Type=10) of the MOB_HO-IND MAC message and transmits the message to the BS. The BS receives the rejection message from the MSS, re-seeks for a target BS and re-transmits the MOB_BSHO-RSP MAC message to the MSS.
3. Neighboring BS Scanning Procedure
FIG. 4 is a diagram illustrating a related art neighboring base station scanning procedure. Referring to FIG. 4, a serving BS connected to an MSS broadcasts information pertaining to neighboring BSs to all MSSs within a BS domain.
The information pertaining to the neighboring BSs is delivered via a MOB-NBR-ADV message. A configuration of the MOB-NBR-ADV message is shown in Table 1.
TABLE 1SyntaxSizeNotesMOB-NBR-ADV_Message_Format( ){Management Message Type = 49 8 bitsOperator ID24 bitsUnique ID assignedto the operatorN_NEIGHBORS 8 bitsFor(j=0;j<N_NEIGHBORS;J++){Neighboring BS-ID48 bitsPhysical Frequency32 bitsConfiguration Change Count 8 bitsHysteresis threshold 8 bitsMAHO report period 8 bitsTVL Encoded Neighbor informationVariableTLV specific}}
Referring to Table 1, the MOB-NBR-ADV message includes a neighboring BS number, a neighboring BS identifier, a neighboring BS frequency, neighboring BS channel information, and other information.
After the MSS receives the MOB-NBR-ADV message, the MSS temporarily stops receiving data transmitted from the serving BS and transmits a MOB-SCN-REQ message for acquiring a time for channel quality measurement from the neighboring BS to the BS. A configuration of the MOB-SCN-REQ message is shown in Table 2.
TABLE 2SyntaxSizeNotesMOB_SCN_REQ_Message_format( ){Management Message Type = 50 8 bitsScan Duration12 bitsUnits are framesReserved 4 bits}
While the MSS measures channel qualities of die neighboring BSs, the serving BS stops transmitting data to the MSS is then able to transmit data after receiving a scanning duration request from the MSS. The serving BS transmits a MOB-SCN-RSP message to the MSS in response to the MOB-SCN-REQ message. A configuration of the MOB-SCN-REQ message is shown in Table 3.
TABLE 3SyntaxSizeNotesMOB_SCN_RSP_Message_format( ){Management Message Type = 50 8 bitsScan Duration12 bitsUnits are framesStart Frame 4 bits}
As shown in the MOB-SCN-REQ message, the BS secures the scanning duration requested by the MSS, and the MSS starts the BS scanning after standing by for a start frame time.
The related art neighboring BS scanning method has several problems and disadvantages. In order to provide a seamless service from the BS, a moving MSS may request the handover. Alternatively, the BS may request the handover in order to balance its load. If the BS requests the handover, the MSS performs scanning on neighboring BSs to decide whether to perform the handover. If the handover is needed, the neighboring BSs must be continuously scanned. However, in order to obtain channel quality information in the related art method, the MSS must transmit the scanning request message and receive The corresponding scanning response message each time. Hence, the MSS continually occupies limited radio resources and unnecessarily consumes power for message transmission and reception.